Main Article

Our body has two different immune systems for defending against disease-causing microbes:

  1. The innate immune system, that works "out of the box" to defend against certain common pathogens. We are born with a fully functional innate immune system; however it has limitations.
  2. The acquired immune system, that "learns" to cope with additional threats that are not adequately addressed by the innate immune system.

When we are exposed to a pathogen that the innate immune system cannot immediately dispatch, our acquired (also called "adaptive") immune system gets called into action, as it recognizes some "non-self" protein in our body that is presumably part of an invading pathogen. Over a period of about a week, our acquired immune system "learns" to produce what are called immunoglobulins (Ig) such as IgM, IgG, IgA, and IgE that "lock on" to these particular non-self proteins and activate the immune system to destroy the invading pathogen. This is why many illnesses last about a week - that is how long it takes for the acquired immune system to learn to mount an attack against the invading pathogen.

But the good news about the acquired immune system is that IT REMEMBERS the what the invading pathogen "looked like, tasted like, and smelled like" (in other words, the particular protein patterns found on the invading pathogen). In the event that the person is re-infected by the same invading pathogen, the acquired immune system does not need a week to ramp-up - it reacts almost instantly, and the invading pathogen is dispatched before it has a chance to cause any harm.

In general, the acquired immune system "remembers" for many years - this is called naturally acquired immunity! Once you have had a certain disease, in many cases, you have many years of immunity. It helps your acquired immune system to "remember" if it is periodically reexposed to the invading pathogen from time-to-time.

Unfortunately, many pathogens change or mutate over time, to the extent that our acquired immune system has difficulty recognizing the newly mutated forms of the pathogens. This is a particular problem for certain pathogens, such as influenza ("flu").

Vaccinations attempt to train the acquired immune system to recognize pathogens that the patient has not yet been exposed to. This works in many cases, but has its limitations. The main limitation of vaccinations is that they do not present to the acquired immune system the "full experience" of actually having the disease, so the acquired immune system does not learn to recognize all aspects of the pathogen. Therefore, the acquired immune system's response to the pathogen may be less full-throated, and the benefit of vaccination is less stable in the face of mutation than the learned benefit of actually experiencing the disease.

In summary, anyone who has survived a particular infection, e.g. COVID-19, is expected to have a stronger future immunity than someone who has just been vaccinated with part of the full immune-challenging experience, and in particular is expected to have a more robust response to mutated "variants," since mutations tend to only affect a portion of the entire pathogen at any one time, while leaving all the other pathogenic markers vulnerable to the learned experience of the acquired immune system.